Import @davidgiven's support for new targets

[johnwbyrd]

It seems that @davidgiven has already gone to the trouble to create llvm-mos linker scripts for half a dozen platforms we don't currently support. If we asked him nicely, I bet he would let us modify and repurpose those scripts for llvm-mos-sdk:

https://github.com/davidgiven/cpm65/tree/master/src/arch

This seems like a low-effort, high-impact way of supporting for 5 or 6 more platforms.

[asiekierka]

• apple2e
• atari800 => We already have a port.
• bbcmicro
• commodore => We already have a port.
• neo6502 => [neo6502] New port. #323 (from-scratch implementation)
• oric
• sorbus
• x16 => We already have a port.